It is often necessary to operate linear and area array solid state imagers in environments containing high energy radiation, such as gamma and/or proton radiation. Such radiation is known to have degrading effects on semiconductor circuit performance and reliability (See for example, J. Janesick et al, "The Effects of Proton Damage on Charge-Coupled Devices", SPIE/SPSE's Electronic Imaging Science and Technology Conference, "Charge-Coupled Devices and Solid State Optical Sensors II", Vol. 1147, San Jose Convention Center, Feb. 24, 1991 and G. R. Hopkinson, "Cobalt 60 and Proton Radiation Effects on Large Format, 2-D, CCD Arrays for an Earth Imaging Application", 1992 IEEE NSRE Conference, New Orleans, La., Jul. 13-17, 1992 and submitted to IEEE Transactions on Nuclear Science, NS-38 (6), 1992). These radiation effects are of particular concern in environments where no service can be performed to repair the damaged semiconductor components. One such environment is the space occupied by low to medium orbit satellites around the planet Earth.
External shielding aboard satellites is used to decrease the dose of radiation received by the imager. Also, special fabrication processes exist which can minimize the degrading effects of radiation on semiconductor components (See for Example, J. Clark, et al, "Radiation Effects on Integrated Circuits", Integrated Circuits for the Space Environment, Space Products News, 6th Edition, Harris Semiconductor Corporation, September 1993). Application of such processes however, is often prohibited due to the resultant degrading effects on imaging performance parameters like quantum efficiency.
Charge transfer inefficiency (CTI) in CCD imagers and the physical mechanisms causing such inefficiencies have been extensively studied (See for example, R. W. Brodersen, et al, "Experimental Characterization of Transfer Efficiency in Charge-Coupled Devices", IEEE Trans. on Electron Devices, Vol. ED-22, No. 2, February 1975 and M. G. Collet, "The influence of Bulk Traps on the Charge Transfer Inefficiency of Bulk Charge-Coupled Devices", IEEE Trans. on Electron Devices, Vol. ED-23, No. 2, February 1976). These studies have shown that the inefficiency in charge transfer can be proportional to the amount of charge being transferred (i.e. proportional loss), or occurs only at or below certain charge levels (i.e. fixed loss), or both.